Download, install and update these browsers from Mozilla:
 - Firefox Browser (Fennec Release)
 - Firefox Focus
 - Firefox Klar
 - Firefox Lite
 - Firefox Preview (Fenix).

FFUpdater will check periodically for updates and will display a notification when an update is available. This feature itself can be disabled and the check frequency can be changed.

Security measures:
 - only HTTPS connections
 - check fingerprint of the downloaded file
 - check fingerprint of the installed app

You can find the APK certificate fingerprints on multiple website - e.g. apkmirror.com
I did my best to make the app as secure as possible - feel free to double-check it in the source code.

Important: I removed Fennec Beta and Fennec Nightly because they are outdated - I will add Fenix Beta and Fenix Nightly soon.